listen

The listener can be added to an object once, and if it is one more time, its key will be returned.
Notice that if the existing listener is a one-off (added using listenOnce),
it will cease to be such after calling the listen() method.

setup

It can be a JSON object or a special value that setups instances of descendant classes.
Note: this method only changes element properties if they are supposed to be changed by the config value -
it doesn't reset other properties to their defaults.